Control rod
From Wikipedia, the free encyclopedia
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium. Chemical elements with a sufficiently high capture cross section for neutrons include silver, indium and cadmium. Other elements that can be used include boron, cobalt, hafnium, gadolinium, and europium. Because these elements have different capture cross sections for neutrons of varying energies the compositions of the control rods must be designed for the neutron spectrum of the reactor it is supposed to control. (Light water reactors (BWR, PWR) operate with "thermal" neutrons, breeder reactors with "fast" neutrons.)
Control rods are usually combined into control rod assemblies - typically 20 rods for a commercial PWR assembly - and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to control the neutron flux - increase or decrease the number of neutrons which will split further uranium atoms. This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricity produced.
Control rods often stand vertically within the core. In pressurised water reactors, (PWR) they are inserted from above, the control rod drive mechanisms being mounted on the reactor pressure vessel head. Due to the necessity of a steam dryer above the core of a boiling water reactor (BWR) this design requires insertion of the control rods from underneath the core. The control rods are partially removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor.
Usually there are also other means of controlling reactivity: In the PWR design a soluble neutron absorber (boric acid) is added to the reactor coolant allowing the complete extraction of the control rods during stationary power operation ensuring an even power and flux distribution over the entire core. This chemical shim, along with the use of burnable neutron poisons within the fuel pellets, is used to assist regulation of the long term reactivity of the core,[1] while the control rods are used for rapid changes to the reactor power (e.g. shutdown and start up). Operators of BWRs use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps. (Increase in coolant flow through the core improves the removal of steam bubbles, increasing the density of the coolant/moderator)
In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. A notable exception to this fail-safe mode of operation is the BWR which requires the hydraulical insertion of control rods in the event of an emergency shut-down, using water from a special tank that is under high nitrogen pressure. Quickly shutting down a reactor in this way is called scramming the reactor.
Originally the control rods hung above the reactor, suspended by a rope. In an emergency a person assigned to the job would take a fire axe and cut the rope, allowing the rods in fall into the reactor and stop the fission. At some point the title of the person assigned this duty was given as SCRAM, or Safety Control Rod Ax Man (although this may be a backronym). This term continues in use today as the phrase "scram" to describe a shutting down of a reactor by dropping the control rods.
Mismanagement or failure of control rods was a cause or an aggravating factor of many nuclear accidents, including the SL-1 explosion.
Homogenous neutron absorbers have often been used to manage criticality accidents which involve aqueous solutions of fissile metals, in several such accidents either borax (sodium borate) or a cadmium compound has been added to the system. The cadium can be added as a metal to nitric acid solutions of fissile material, the corrosion of the cadmium in the acid will then generate cadmium nitrate in situ.
In carbon dioxide cooled reactors such as the AGR if the solid control rods were to fail to arrest the nuclear reaction, nitrogen gas can be injected into the primary coolant cycle. This is because nitrogen has a larger absorption cross section for neutrons than carbon or oxygen, hence the core would then become less reactive.
As the neutron energy increases the neutron cross section of most isotopes decreases, it is interesting to note that 10B is the boron isotope which is responsible for the majority of the neutron absorption. Boron containing materials can be used as neutron shields to reduce the activation of objects close to a reactor core.
